The present invention relates to improvement of a high frequency filter utilized in VHF, UHF, and microwave frequency bands.
The present filter can be utilized in radio communication apparatus in said frequency area for preventing interference from adjacent communication channels. Preferably, the present filter is utilized in the antenna circuit of a mobile communication system.
For that purpose, a filter employing a coaxial line type resonator has been utilized. Said resonator has an internal conductor, a cylindrical external coaxial conductor and a dielectric body between those conductors. The dielectric body is used for the purpose of reducing the size of a resonator and/or a filter.
FIG. 1(A) and FIG. 1(B) show the structure of a prior coaxial line type resonator utilized in a prior high frequency filter, in which FIG. 1(A) is a vertical sectional view, and FIG. 1(B) is a plane sectional view. In those figures, the reference numeral 1 is an inner conductor, 2 is a cylindrical external conductor arranged coaxially with the inner conductor 2. One extreme end of the inner conductor 1 is short-circuited with the external conductor 2, and the other extreme end of the inner conductor 1 is open. In this type of resonator, the following formulae are satisfied, where .epsilon..sub.r is relative dielectric constant of dielectric body 3, .lambda..sub.g is the wavelength in a coaxial line, .lambda..sub.O is the wavelength in free space, f.sub.O is the resonant frequency, C is the light velocity in free space, and l is the length of the resonator, and said length is the same as the length of the inner conductor 1. ##EQU1## As apparent from the above formulas, the larger the relative dielectric constant .epsilon..sub.r is, the shorter the length (l) of the resonator can be, and the size of the resonator can be reduced. On the other hand, supposing that the dielectric loss by the dielectric body 3 is constant, the radius (b) of the external conductor 2 is obtained by the unloaded Q (which is designated as Q.sub.u). When the value of (b) is small, the value Q.sub.u also becomes small and the electrical loss is increased, so that radius (b) of the external conductor 2 is determined by the allowable loss. Further, the radius (a) of the inner conductor 1 is determined so that b/a=3.6 in which the value Q.sub.u becomes maximum.
FIG. 2(A), and FIG. 2(B) show a prior high frequency filter utilizing three resonators shown in FIG. 1(A) and FIG. 1(B), in which FIG. 2(A) is the plane sectional view, and FIG. 2(B) is the vertical cross-sectional view, the reference numeral 1 is an inner conductor, 2 is an outer conductor, and 3 is a dielectric body. The reference numeral 4 is a loop antenna for coupling the filter to the external connector 6. 5 is a window provided on the wall 5a which is a part of the outer conductor 2 for connection between the adjacent resonators.
However, a high frequency filter utilizing the above mentioned coaxial resonator dielectric body has the disadvantage that the manufacturing cost of the same is considerably high. The main reason for the high cost is the presence of an air cap between the inner conductor 1 and the dielectric body 3, and between the outer conductor 2 and the dielectric body 3. Of course, it is desirable that said air gap does not exist for proper operation of the filter.
FIG. 3(A) and FIG. 3(B) show the practical structure of a filter, in which an air gap 1a exists between the inner conductor 1 and the dielectric body 3, and an air gap 2a exists between the outer conductor 2 and the dielectric body 3. Those air gaps 1a and 2a are inevitable in a prior filter manufacturing system, in which a hollow cylindrical dielectric body 3 made of ceramics is inserted in the ring shaped space between the inner conductor 1 and the outer conductor 2. The presence of the air gaps 1a and 2a reduce the effective dielectric constant .epsilon..sub.r of the dielectric body 3, and further, the small drift or change of the width of the air gaps 1a and 2a changes the resonance frequency f.sub.O of a resonator considerably. Those matters will be mathematically analyzed in accordance with FIG. 4 and FIG. 5.
FIG. 4 shows the mathematical model of a resonator, in which (a) is the racius of the inner conductor 1, (b) is the radius of the outer conductor 2, .DELTA.a is the width of the inside air gap 1a, .DELTA.b is the width of the outside air gap 2a, the area I and III are air spaces provided by said air gaps 1a and 2a, respectively, and the area II is the space occupied by the dielectric body 3.
The change .DELTA.f of the resonance frequency f.sub.O of the resonator in FIG. 4 is given by the formula (2), providing that the change of the inductance (L) of the l portion of the coaxial cable by the presence of the air gaps is neglected. ##EQU2## For example, a=2.8 mm, b=10 mm, and .epsilon..sub.r =20 are assumed in the formula (2), the following relationship is satisfied. ##EQU3## As apparent from the above formula (3), the presence of 1% change of the air gaps ##EQU4## due to a manufacturing error in the inner conductor 1, the outer conductor 2 and the dielectric body 3, provides 7.8% of the change of the resonance frequency f.sub.O. According to our experiment in the 900 MH.sub.z band, the presence of 1% of the air gaps provided the change of the resonant frequency in the range of 3%-10%. The change of the resonant frequency f.sub.O depends upon the arrangement of the inner and the outer conductors, that is to say, the arrangement in FIG. 4 provides a larger change of the resonant frequency, and the arrangement in FIG. 5 in which the inner conductor is eccentrically positioned provides the smaller change of the resonant frequency.
In a prior high frequency filter, a conductor screw 7 in FIG. 3 is provided to compensating for the change .DELTA.f of the resonant frequency f.sub.O. For instance, the insertion of the conductor screw 7 by 10 mm in the filter having the size a=2.8 mm, b=10 mm, .epsilon..sub.r =20 and the radius a.sub.1 of the screw 7 is 2 mm, provides a 70 MH.sub.z change of the resonant frequency in the 900 MH.sub.z band. In this case, the formula (4) is satisfied from the above formula (3) and assuming that the ratio .DELTA.a; .DELTA.b=1;3, then the allowable errors are 2.DELTA.a=30 .mu.m, and 2.DELTA.b=90 .mu.m. ##EQU5## As apparent from the above mathematical analysis, a prior high frequency filter having coaxial cable type filters leaves small tolerance for manufacturing error.
In order to overcome the above drawback, the improvement of a filter has been proposed, in which the air gaps 1a and 2a are eliminated. According to said improvement, thin film electrodes are either printed on the outer and the inner surfaces of the dielectric body 3, or connected to the outer and the inner conductors by conductive adhesives. However, those proposals have the disadvantage that the effective Q.sub.u of a resonator is considerably reduced due to the resistance loss by the printed electrodes and/or the adhesives.
Accordingly, the tolerance for manufacturing error in a prior high frequency filter is very severe, therefor, the manufacturing cost of a prior filter is high.